Update
When Christopher Pratt and Jérémie Beyou invited sponsors and their guests along for the speed runs during the Défi Azimut they may not quite have been expecting this, but they will certainly have gone back babbling about having the time of their life. Charal is just getting faster and faster but as the newer boats sort themselves out and become more consistent, expect Beyou to push even harder to maintain his psychological advantage. In the meantime, surely it would be crazy if having got this far with offshore foiling, the Imoca rule is not changed in time to permit the use of rudder foils for The Ocean Race. Anything else would now just look bizarre
COMPLEXITY – Jack Griffin Foiling came to the America’s Cup in 2012, when Emirates Team New Zealand flew their AC72 on the fourth day of sailing it. Actually, it had come in 2011, when ETNZ were secretly testing their smaller ‘Waka’ test boat, towing it behind a motorboat on Lake Arapuni. Crude test platforms have long since given way to sophisticated simulators. Cup teams now form technical partnerships with aircraft manufacturers and aerospace systems suppliers. We all need to master some new vocabulary. Nick Holroyd, head
of design for Ineos Team UK, has said that one of the big benefits of their test boat was helping them develop a common language between the sailors and the design team. Presumably all the teams now use a lot of aircraft terms. Like aircraft, foiling yachts can experience ‘Dutch roll’ when yaw and roll (formerly called heel) get out of phase. ‘Flight controller’ is now a crew position on a yacht. Rudders have horizontal stabilisers, not elevators, as we mistakenly called them during AC34. The simulators predict static and dynamic stability modes and
the sailors and yacht instrumentation will report back how closely the physical world of the yacht agrees with the virtual world of the simulator. The simulators will improve with the data sets from the physical world. Simulators will also get a workout analysing com- petitors. It’s a fairly straightforward process to develop a model of a competitor’s yacht from photos. Running the model of the com- petitor’s boat through your simulator will give clues about how they optimised their design, and may point out biases in your own tools. Unlike most aircraft, the AC72 and AC50 catamarans raked the
entire foil to vary the angle of attack of the foil wings. The AC75s have flaps on the foil wings which vary both the foil shape and angle of attack, like aircraft. The foil arms themselves are not allowed to be raked, only canted. Teams are allowed to build six foil wings and 20 flaps. Developing the AC75 class rule gave ETNZ a head start in design
12 SEAHORSE
and simulation. Interestingly, they are the only team to launch without a bulb at the intersection of the foil arm and the foil wing. ETNZ also launched (as did Ineos) with different port and starboard wings – straight on one side and anhedral on the other. ETNZ’s foil wings appear to have the largest surface area of the
four teams. The foils on American Magic’s test boat did not have a bulb, but their AC75’s foils have large bulbs and anhedral. Luna Rossa appear to have the same-shaped anhedral foil wings with large bulbs on both sides but a small pod outboard on the starboard foil may contain an actuator for the foil flaps. The flap-actuation systems must be quite complex. Testing and
optimising them will surely take a lot of processor cycles – both in the simulators and in the sailors’ brains. Ineos Team UK’s Britannia has anhedral wings with fences on the underside and smaller bulbs than Luna Rossa or American Magic’s Defiant. It seems that every- one’s simulators have different assumptions about cavitation and how to optimise lift-off, high-speed flight and manoeuvres. AC75 rudders of course have horizontal stabilisers, but elevators
or further moveable parts are not allowed by the class rule. Yaw dampers are also not allowed – the only yaw input to the rudder must come from the helm. On aircraft a yaw damper automatically makes slight rudder adjustments to avoid Dutch roll. Some aircraft rudders move less for a given input at higher speeds, since more yaw moment is produced by a given rudder angle. The AC75 class rule does not seem to forbid this – perhaps that’s a question for the Rules Committee. The control systems for the AC75s have taken complexity to a
completely new level. Patrizio Bertelli has cited this complexity as a major reason for having so few challengers. Hydraulics, electronics, even the wiring looms add to the complexity and cost. Design teams are now much more multi-disciplinary; a far cry from 15 years ago when naval architecture, sail design and structural engineering were the major disciplines. CFD software is still part of the game, but now simulators also need to process data from many more state
VINCENT CURUTCHET/ALEA
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